$C_D$ is a form factor variable, constructed to be as independent as possible of all the factors that contribute to aerodynamic forces. Two dimensionless flow parameters do influence outcomes of aerodynamic measurements:
- The Mach number, which quantifies compressibility of air.
- The Reynolds number, which accounts for viscosity/inertia effects in the boundary layer.
On our Aviation Stack, most airflow considerations are relevant to long slender bodies at flight speed (between take-off and cruise speed), meaning the range of Re is reasonably limited. For a car this may be a different matter.
Re can be illustrated with the golf ball effect: the wake around the golf ball without dimples is what happens at lower Re, where viscosity effects dominate. At higher Re the boundary layer becomes turbulent and can follow the contours of the ball better, which creates less pressure drag.
Flight at lower Re takes place in gliders, and there are measurement data on glider airfoils at various Re. For instance in this book, Low Speed Airfoil Data by M. Selig, figure above is from page 68. It clearly shows how $C_D$ values change with Re at constant $C_L$. Note that differences at higher values are very small, at lower values very large.
This can only be possible if Cd is different for different speeds.
Yes indeed, at lower Re with laminar boundary layer we'll find a different $C_D$ than at high Re with mainly turbulent boundary layer.
Cars are short bodies moving entirely in ground effect, and usually the frontal area is used as the reference for $C_D$ as the form factor variable. Why? Because short streamline bodies dominant aerodynamic drag factor is the pressure drag, including suction pressure from the separated flow at the rear. And this flow separation takes place only at higher speeds, especially with the gently sloping rear of the Porsche - which reduces drag, but (unfortunately for a car) also produces local lift.
Pics above are from this very comprehensive master's thesis and show how the spoiler usually reduces drag: by reducing turbulence in the wake, thereby increasing the local air pressure. The thesis also mentions Porsche, and the way that OPs referenced spoiler works:
Companies such as Porsche, Bugatti or Mercedes have been using different technologies for spoilers and trying to maximizing the efficiency of it by eliminating the side effects in low speeds and increasing the advantages on high speeds. One of the most commonly used features is to have a hydraulic wing style spoiler at the rear end of vehicle that raises or lower at certain speeds to maintain down force on the backside of vehicle or to create air brake. This feature has been used mostly for safe driving. Spoiler deployment operation is usually automatic. The software operates the spoiler and fixes it in the certain height depends on the vehicle speed but the driver through a button in the cabin can also operate it. For instance, hydraulic spoiler that has been used in Bugatti Veyron comes up at high speeds to hold the car on the road better by creating down force. When the car reaches 220 km/h (140 mph), small hydraulic spoiler deploys from the rear bodywork and a wing extends about a foot. This configuration produces substantial down force, provides up to 330 pounds in front and 440 in the rear , which helps holding the car to the road in extreme speeds.